In the art of arc welding, it has been discovered that welding in narrow grooves can be done very effectively by pulsing the current in the arc. This is typically done by allowing the flow of current in response to line voltages to occur only for short periods toward the end of each half cycle of the supply voltage. One of the difficulties attending this method, however, is the fact that it is particularly sensitive to line-voltage variations. It is desirable in these applications that the average current in each pulse be equal, but due to power supply variations, the average current produced by this method varies unless preventative steps are taken.
The obvious response to this problem is to regulate the supply voltage, thereby maintaining a constant average current once the duty cycle is set. Unfortunately, means for accomplishing regulation of the AC supplies tend to be rather elaborate and expensive. An example is the motor-driven variable transformer. This is obviously rather elaborate and in addition has a slow response. The slow response, of course, makes the variable transformer relatively ineffective at insuring that each pulse has the same average current. A faster response is afforded by the constant-voltage transformer, but it also is large and expensive and tends to limit its usefulness to equipment that can tolerate its current-limiting action. As a result, what is needed is a fastacting and inexpensive means for compensating for line-voltage variations.